Probably the convenience of paying multiple creators with one credit card transaction, and one login. I guess you could call this a network effect. YT had/has various direct payment options, but they are confusing and never made as much sense as Patreon's model, which allows per-month or per-creation pledges. Strange since YT's existing infra would seem a big advantage in doing exactly what Pareon has.

I think it varies a lot. Some Patreon creators use it as a sort of "tip jar", where the rewards for various tiers are relatively minimal compared to the main product that is given away for free. This is very common for web comics, and I'm guessing also common for Youtubers. In these cases, the main "reward" is helping to ensure that the creator can continue producing the content they're releasing for free. Other creators use Patreon as the medium by which to deliver their content, and only patrons hace access to it. So this is more like a traditional subscription model than a tip jar. And of course there are various gradations in between, and probably other models as well.

So because Patreon is used in such a wide range of ways, It's hard to make generalizations about all of it. And by that same token it's also hard for Patreon to make changes in a way that doesn't interfere with the way someone is using it. (Not that this in any way excuses their numerous blunders.)

The nice thing about Patreon is it explicitly is structured for tiered subscription support and removes the need for creators to do any payment system managing.

It's not a discovery platform. Essentially (and I know Jack Conte would like to evolve this but it already works without such an evolution) there's no reason to even be on a Patreon page unless you were already fixing to pay the creator. That makes it pretty self-selecting.

[Not op] There are a few reasons, but two big ones are being directly supporting the creator (YT requires you use memberships which are flat-rate and not user-adjustable) and getting perks for pledging (like Discord access, access to pre-release work, etc).